Clutches are widely used to couple driven rotary shafts to driving rotary shafts; i.e., to couple a load to a source of power. Because in most instances a driven shaft is stationary and a driving shaft is moving, slippage occurs between engaging surfaces of a driven member of the clutch and of a driving member of the clutch while friction between these surfaces accelerates the driven shaft to the speed of the driving shaft during engagement of a clutch. The relative motion of slippage in the presence of friction generates heat. If the load is large (e.g., a hydraulic system starting against pressure or a device high in mass moment of inertia), a great deal of heat may be generated.
A disc clutch is a type of clutch often provided in both stationary and mobile equipment (e.g., industrial machinery, marine drives, agricultural power takeoffs, etc.). A disk clutch includes at least one generally disc-shaped driving member (termed herein "pressure plate" and coupled to an input, or driving, shaft) and at least one generally disc-shaped driven member (termed herein "clutch plate" and coupled to an output, or driven, shaft). An actuator located within or upon the clutch forces the pressure plate against the clutch plate when clutch engagement is desired, whereupon frictional drag of pressure plate upon clutch plate accelerates the driven shaft (connected to a load) up to the speed of the driving shaft (connected to a prime mover such as an engine or a motor).
In some such instances (e.g., automotive engine-to-transmission coupling and uncoupling), convective cooling by ambient air is sufficient to carry away the heat of engagement. In other instances (e.g., an agricultural tractor power takeoff, or PTO, shaft) a clutch smaller in diameter than that of an automotive engine flywheel is desired and, to transmit the torque, a plurality of pressure plates and clutch plates is needed. Such clutches generally do not dissipate heat sufficiently rapidly to avoid damage to seals, bearings and other components, particularly when operating under harsh circumstances such a duty cycle including frequent engagements with a high-inertia load in a high ambient temperature.
Many stationary and mobile clutches are therefore configured as multidisk clutches, including a plurality of pressure plates and clutch plates and, typically, a hydraulic piston within a sealed chamber to force the plates into engagement with each other. These are often "wet" clutches; i.e., the pressure plates and clutch plates are housed within a second chamber contiguous with the first chamber, and a liquid lubricant with coolant properties (termed "lube fluid" herein) is introduced to the second chamber in, typically, a small quantity sufficient to lubricate and cool bearings and seals but not enough to create a large hydrodynamic drag upon the pressure and/or clutch plates and thereby a parasitic power loss. To assist in dissipating small amounts of heat generated by bearings and seals while the clutch is not in the process of being engaged, the small quantity of lube fluid is constantly replaced by slowly introducing cooled fluid while simultaneously removing a similar flow rate of warmed fluid.
In some instances, however, including some agricultural applications such as a tractor PTO clutch, the amount of heat generated during engagement is so large that the small quantity and flow rate of lube fluid is inadequate to prevent overheating of the clutch. It is then desirable to increase the quantity and flow rate of lube fluid during engagement of the clutch.
It is known to provide a valved relationship between a pressure plate piston chamber and a chamber housing pressure and clutch plates, often utilizing passages machined within a shaft. Such valving relationships do not, however, provide an anticipatory, or "look-ahead", or feed-forward control condition with the result that a rapid engagement of the clutch may prevent the added lube fluid from reaching the interior of the second chamber in time to prevent excessive heat buildup. Moreover, passages internal to a shaft weaken the shaft, particularly when they are configured as intersecting drilled holes having sharp corners which serve as stress risers, and also when located in the region of a shaft keyseat.
It is also known to provide an electronic control system to ensure that actuating fluid is not applied at pressure to the clutch piston until lube fluid flow rate and/or level have increased sufficiently to dissipate the heat of engagement. Such a control system may be expensive, however, particularly when the costs of related sensors, actuators and/or valves, and signal conditioning and/or converting apparatus are considered. Further, the higher component count of such a system may negatively impact system reliability, particularly when operating in a harsh environment.
It would be advantageous to provide for a power transmission system to be useable for a mobile work vehicle as well as for a stationary power unit, the power transmission system including a control valve which responds to an electronic ENGAGE signal by increasing lube fluid flow rate from a first rate to a second rate over a predetermined interval of time before increasing clutch piston actuating fluid pressure.
It would further be advantageous to provide for the control valve of such a power transmission system to reduce lube fluid flow rate from the second rate to a predetermined third rate over a second predetermined time interval after increasing lube fluid flow rate from the first rate to the second rate, so that the increased lube fluid flow rate and level exist substantially only while the clutch is in the process of engaging.